Cells of micro-algae are rich in various bioactivity substances such as proteins, amino acids, carbohydrates, vitamins, antibiotics, highly unsaturated fatty acids, polysaccharides, and colorants. This makes micro-algae great resources with high economic value. Some micro-algae possess abilities to produce hydrocarbon (such as Botryococcus braunii), and thus have promising application in field of renewable energy production. Today, as global food and energy crises are becoming more severe, development and utilization of micro-algal resource have exhibited a great significance and economic prospect. Large scale cultivation of micro-algae is normally carried out either in open cultivation systems or in the closed photoreactors. The open cultivation system comprises open-pond, raceway pond, or round shallow pond. It has been successfully applied in the commercial production of Spirulina, Chlorella, and Dunaliella salina due to its simple management and low investment. (Chaumont D., J. Appl. Phycol., 1993, 5:59-604; Bonnin G., Spirulina Production Engineering Handbook, BECCMA ed., Nantes, France, 1992, 140-159; Richmond A., Progress in Physiological Research, Vol. 7, Biopress, Bristol., 1990, 269-330; Borowitzka L. T., Bioresource Technology, 1991, 38:251-252). The closed photobioreactors have different structures, such as airlift reactor, stirred reactor, or tubular reactor, which can be used for producing high value added products (such as medicinal or health products) or used as seed tank for open-pond cultivation (Hu Q., J. Appl. Phycol., 1994, 6:391-396; Carlozzi P., Appl. Microbiol. Biotecnol., 1996, 45:18-23; Lee Y. K., J. Appl. Phycol., 1995, 7(1):45-52; Hu Q., Biotech. Bioeng., 1996, 51(1):51-60; Wohlgeschaffen G. D., J. Appl. Phycol., 1992, 4:25-29).
Micro-algal cells fix carbon dioxide through photosynthesis and carbon comprised more than half of its dry weight. Therefore sufficient carbon source is needed during micro-algae cultivation. Carbon dioxide exists in the form of HCO3—, CO32— and free CO2 in the solution. The ratios of the three carbonate forms vary with the pH value. The detail is shown in FIG. 6.
In large scale open cultivation conditions, the depth of the culture solution is usually kept less than 15-20 cm to insure the sufficient light irradiation for cell growth. In this case, if CO2-containing gas is directly introduced into the culture solution for supplying carbon, the residence time of bubbles in the culture solution is short due to shallow depth of the solution, and the utilization of CO2 is low. For this reason, NaHCO3 is currently the major carbon source used in large scale micro-algae cultivation. But NaHCO3 can not be fully utilized as carbon source during the cultivation. The dissociation and utilization of the HCO3— lead to a continual rise of pH value and unsuitable for micro-algae growth. More than half of the NaHCO3 is turned into unusable Na2CO3 and therefore results in a considerable waste of water and carbon source. This is one of the main reasons for the high cost of micro-algae production. For example, the consumption for producing one ton of Spirulina (based on dry weight) is 8 tons of NaHCO3, 1000 tons of water and 3 tons of nutrient salts.
As analyzed above, the cost of algal cultivation can be reduced dramatically by the direct use of CO2 gas or liquid only if the utilization is greatly improved. According to the estimate, for Spirulina production, the cost of carbon source by using NaHCO3 is 6 times of that by using CO2 for carbon supply (estimate is based on the assumption that all the provided CO2 is completely absorbed by the culture solution). Additionally, CO2 is the optimal carbon source for micro-algae growth. The pH value of the culture solution is kept relatively constant by using CO2, which is beneficial for maintaining desirable culture environment, and allows water to be used repeatedly or for an extended period.